The need for producing variable magnetic fields arises in a number of industries and fields of endeavor, especially in the manufacturing sectors. One example can be found in the semiconductor processing industry.
Sputtering, alternatively called physical vapor deposition (PVD), is a widely used method of depositing layers of metals and related materials in the fabrication of semiconductor integrated circuits. A conventional PVD reactor includes a vacuum chamber sealed to a PVD target composed of the material, usually a metal, to be sputter deposited on a wafer held on a wafer support, or pedestal. A shield held within the chamber protects the chamber wall from the sputtered material and provides the anode grounding plane. A DC power supply negatively biases the target with respect to the shield. In some designs, the pedestal and hence the wafer are left electrically floating.
A gas source supplies a sputtering working gas, typically the chemically inactive gas argon, to the chamber. A vacuum system maintains the chamber at a low pressure, with a typical working gas pressure in some designs at between about 1 and 1000 mTorr.
When the argon is admitted into the chamber, the DC voltage between the target and the shield ignites the argon into a plasma, and the positively charged argon ions are attracted to the negatively charged target. The ions strike the target at a substantial energy and cause target atoms or atomic clusters to be sputtered from the target. Some of the target particles strike the wafer and are thereby deposited on it, thereby forming a film of the target material.
Advances in semiconductor design have placed increasing demands upon sputtering equipment and processes. Some of the problems are associated with contact and via holes in the semiconductor wafers. Sputtering is often used to fill metal into the vias to provide inter-level electrical connections. In advanced integrated circuit designs, the via holes have increased aspect ratios of three and greater.
Such high aspect ratios present a problem for sputtering because some forms of sputtering are not strongly anisotropic, so that the initially sputtered material preferentially deposits at the top of the hole and may bridge it, thus preventing the filling of the bottom of the hole and creating a void in the via metal.
It has become known, however, that deep hole filling can be facilitated by causing a significant fraction of the sputtered particles to be ionized in the plasma between the target and the pedestal. In some designs, the pedestal, even if left electrically floating, develops a DC self-bias, which attracts ionized sputtered particles from the plasma across the plasma sheath adjacent to the pedestal and deep into the hole in the dielectric layer. The effect can be enhanced with additional DC or RF biasing of the pedestal electrode to additionally accelerate the ionized particles towards the wafer, thereby controlling the directionality of sputter deposition.
One method of increasing the percentage of sputtered atoms which are ionized is to position a magnetron behind the target. Some magnetron designs include opposed magnets creating a magnetic field within the chamber in the neighborhood of the magnets. The magnetic field traps electrons and, for charge neutrality, the ion density also increases to form a high-density plasma region within the chamber adjacent to the magnetron. Some magnetrons are designed to be rotated about the center of the target to achieve full coverage in sputtering of the target.
In some processes, however, it is desirable to be able to produce varying magnetic fields. This need can arise when two or more different processes are performed on the same wafer in the same chamber. Also when new processes and systems are being tested, the magnetic field may need to be varied in order to determine the optimum parameters for the system. Electromagnetic coils traditionally have been used as the source of variable magnetic fields. However these devices can be bulky and expensive to manufacture. Moreover, they can generate a great deal of heat, thus requiring the design and installation of expensive cooling systems. Also, they can present a personnel hazard in the form of electrical shocks. For these reasons, it is desirable to develop a system of permanent magnets which is capable of producing a variable magnetic field.